New mobile communication systems are continuously being developed and more and more functions tend to be integrated into the mobile phone terminals, causing increased power consumption in the terminals. It is difficult to improve the capacity of batteries because the size of the batteries is reduced together with the reduction of size of mobile phones. It is thus of great importance to find ways of reducing the power consumption in mobile phone terminals.
One of the more power consuming components in a mobile phone is the power amplifier in the radio transmitter. A reduction of the power consumption of the power amplifier would be a good contribution to the reduction of the power consumption of the mobile phone.
The output power from and the efficiency of a power amplifier (PA) are strongly affected by the input power to the PA and the impedance of components loading the PA. The optimal value of the impedance of the components loading the PA varies in dependence on the output power from the PA.
A prior art set up for a power amplifier in a mobile phone is schematically illustrated in FIG. 1. A transmission signal is fed to the input 3 of a power amplifier 1. A control signal 4 is fed to the power amplifier 1 to specify what output power the signal is to be amplified to. The amplified signal is then transmitted through an antenna element 2. There is often provided both filters and switches between the power amplifier and the antenna element (not illustrated). The combined components after the power amplifier constitute the impedance loading the power amplifier.
Traditionally the impedance loading the PA is chosen to optimize the PA for maximal power output from the PA. A PA outputting lower than maximal power will not work optimally efficient. However, this is ignored as the inefficient lower power output consumes less power than the efficient maximal power output.
In EP 1093222 assigned to Matsushita Electric Industrial Co, a variable load circuit is utilized to compensate for varying loads loading a power amplifier, thus to load the power amplifier with a constant load. The variable load circuit provided between the output of the power amplifier and the feeding portion of an antenna element introduces extra costs and is also bandwidth limiting.
In U.S. Pat. No. 5,673,287 assigned to Motorola, a dual mode amplifier network enables a radiotelephone to operate in two different transmission modes, one analog and one digital, with one power amplifier. This power amplifier is nevertheless only optimized for maximal transmission power, even if for two transmission modes.